The Steps Involved in the Processes of Transcription and Messenger RNA Processing in Eukaryotic Cells
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The Steps Involved in the Processes of Transcription and Messenger RNA Processing in Eukaryotic Cells

This article details the processes of transcription and messenger RNA processing that occur during gene expression in eukaryotic cells.
           Transcription is the first major step of gene expression in which a small portion of the genetic information stored in double-stranded DNA is copied and transferred to a single-stranded messenger RNA (mRNA) molecule. In eukaryotes, transcription is initiated when a general transcription factor known as TFIID binds to a special DNA sequence immediately upstream from the start site of transcription called the promoter. The region of the promoter to which TFIID binds is rich in adenine and thyamine nucleotides and is thus referred to as the TATA box. Once TFIID is bound to the TATA box at the promoter, it causes a conformational shift in the DNA such that it becomes more accessible to additional general transcription factors that subsequently assemble in close proximity to TFIID. The other general transcription factors that are attracted to the promoter in response to the binding of TFIID to the TATA box include TFIIB which binds directly to TFIID, TFIIE, and TFIIH. Finally, an enzyme called RNA polymerase II in association with TFIIF binds to the promoter thereby completing the transcription initiation complex. When the transcription initiation complex has fully formed, the kinase domain of TFIIH phosphorylates the “tail” domain of RNA polymerase II so that the enzyme can begin to transcribe the DNA sequence into mRNA.

            After RNA polymerase II is successfully launched from the transcription initiation complex, it uses one strand of the DNA duplex as a template from which to lay down complementary RNA nucleotides in a 5′ to 3′ direction. While RNA polymerase II is transcribing the selected gene, the elongating mRNA transcript is simultaneously subjected to a series of RNA processing events that occur only in eukaryotes. First, a specialized guanine cap is added to the 5′ end of the mRNA transcript once it has reached a length of approximately 25 nucleotides. Next, small nuclear ribonucleoprotein particles (snRNP’s) embedded within spliceosomes and other small nuclear RNAs (snRNAs) facilitate the removal of non-coding, intervening sequences known as introns from the mRNA transcript. This RNA processing step is necessary because some relatively extensive nucleotide sequences that fall within the transcribed region of a gene do not encode information for the incorporation of amino acids into a polypeptide chain. Consequently, the introns are spliced out of the mRNA molecule such that the transcript contains only an uninterrupted collection of protein-coding nucleotide sequences called exons. Spliceosomes can link specific exons together in different combinations using a technique called alternative splicing thereby enabling multiple proteins to be assembled from the information encoded in a single gene. Finally, the last step of RNA processing is the addition of a poly-A tail to the 3′ end of the mRNA transcript. This poly-A tail is a long string of adenine nucleotides which is thought to determine the lifetime of an mRNA molecule once it is exported into the cytoplasm of a cell.

 Sources:

Alberts, Bruce, et al. Essential Cell Biology. 3rd ed. New York: Garland Science/Taylor and Francis Group, 2010. Print.

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